In a gas turbine burner a fuel is burned to produce hot pressurised exhaust gases which are then fed to a turbine stage where they, while expanding and cooling, transfer momentum to turbine blades thereby imposing a rotational movement on a turbine rotor. Mechanical power of the turbine rotor can then be used to drive a generator for producing electrical power or to drive a machine. However, burning the fuel leads to a number of undesired pollutants in the exhaust gas which can cause damage to the environment. Therefore, it takes considerable effort to keep the pollutants as low as possible. One kind of pollutant is nitrogen oxide (NOx). The rate of formation of nitrogen oxide depends exponentially on the temperature of the combustion flame. It is therefore attempted to reduce the temperature over the combustion flame in order to keep the formation of nitrogen oxide as low as possible.
There are two main measures by which reduction of the temperature of the combustion flame is achievable. The first is to use a lean stoichiometry with a fine distribution of fuel in the air, generating a fuel/air mixture with a low fuel fraction. The relatively small fraction of fuel leads to a combustion flame with a low temperature. The second measure is to provide a thorough mixing of fuel and air before the combustion takes place. The better the mixing, the more uniformly distributed the fuel is in the combustion zone and the fewer regions exist where the fuel concentration is significantly higher than average. This helps to prevent hotspots in the combustion zone which would arise from local maxima in the fuel/air mixing ratio. With a high local fuel/air concentration the temperature will rise in that local area and so does as a result also the NOx in the exhaust.
Modern gas turbine engines therefore use the concept of premixing air and fuel in lean stoichiometry before the combustion of the fuel/air mixture. Usually the pre-mixing takes place by injecting fuel into an air stream in a swirling zone of a combustor which is located upstream from the combustion zone. The swirling leads to a mixing of fuel and air before the mixture enters the combustion zone.
GB 2334087 A is addressing the specific problem to improve the fuel to air ratio during start-up of a “lean burn” combustor. A combustor comprises a swirler with at least one restrictor to restrict the flow of fluid through the combustor. Preferably the restrictor may be biased or switched between restricting and non-restricting positions depending on the pressure of the airflow. This may optimise the fuel/air mixture. On the other hand the restrictors may cause dead zones in which the airflow is unstable and stagnant with a possibility that flashbacks may occur.
From U.S. Pat. No. 6,192,669 B1 it is known to arrange a plurality of burners, operatively connected to each other, in such a way, so that a swirl flow is initiated in a common combustion chamber which ensures the stability of the flame front. This is advantageous because this may to low pollutant emissions, e.g. NOx, at part load.
US patent application US 2006/0257807 A1 discloses a combustor with a swirler. Circular mixing ducts may be applied to a radial type swirler. This is advantageous due to the absence of corners where excessive fuel could get trapped.
With respect to the mentioned state of the art it is an object of the invention to provide a swirler, in particular a swirler in a gas turbine combustion chamber, a combustion chamber equipped with such a swirler, and a gas turbine having a plurality of such combustion chambers, so that mixing fuel and air in a swirling area is improved by providing a homogenous fuel/air mixture, especially at all possible loads of the gas turbine.